Distributed Raman amplification is a technique useful to achieve amplification gain in an optical fiber by injecting strong light power, called a “pump”, at a proper wavelength. It is common to employ multi-wavelength light pump sources in order to achieve broad spectrum light amplification suitable for dense wavelength division multiplexed (DWDM) applications. The light pump source needs to be set to a proper power level and unbalance, whether for light pump at a single wavelength or at each of multiple wavelengths in order to achieve a desired gain level, gain flatness and gain tilt. There are approaches heretofore known to set the power level of a Raman light pump source.
One approach is to evaluate the Raman pump power level and unbalance through trial and error. For example, a full DWDM “comb” is transmitted through an amplified optical fiber. The received spectrum is measured and the power level and unbalance are changed continuously until the desired gain, tilt and flatness have been achieved. This procedure may be performed manually by a human operator or automatically.
Another approach involves a complete characterization of an optical fiber span in terms of a Raman gain coefficient, spectral attenuation and longitudinal loss profile along the propagation direction. Data related to these parameters are supplied as input to a numerical model that simulates the Raman amplification process and thus is used to evaluate the proper power levels and unbalance. Values for the power levels and unbalance so determined are applied to the Raman light pump source and are subsequently finely tuned using a spectrally resolved measurement of a large DWDM comb transmitted through the optical fiber.